Help in setting up FTP server

I was wondering if anybody has any experience setting up an FTP server?  I'm currently using G6 FTP server as the software.  Here's my situation...users can log in but they get no file listing when they log in.  The FTP server works on the internal network though.  Also, perhaps my setup can give anybody a clue as to why I'm having this problem.

Network consists of a Linksys 4-port router and an Apple Airport station on the uplink port.  There are 5 clients (3 have wireless access and 2 are CAT-5 connected to the router) and all are acquiring their IPs through DHCP.  The G6 software doesn't work unless I have port forwarding specified in the router config. (i.e. port 234 for IP address 192.168.1.102). For some reason the software always tells me that port 21 is being used already.  

Now, here's the problem.  I read the help file on port forwarding for the router. It says that I must have the DHCP feature off in order to use the port forwarding feature of the router.  However, I do not want to go to all 5 computers and give them a static IP.  The reason for this is because 3 of the 5 are not mine.  So, I'm wondering how can I get this FTP server to work while using DHCP for the router?  Does anybody have any experience with this or how about recommending other software that might work?  Thanks for the help!